Light modulation system using phase controlled synchronous motors



Oct. 19, 1965 D. M. NEALE LIGHT MODULATION SYSTEM USING PHASE CONTROLLED SYNCHRONOUS MOTORS Filed April 26, 1963 14/11/ 5 T FIG l 4Mp I O 2 Sheets-Sheet l FIG.4.

Oct. 19, 1965 D. M. NEALE 3,213,377

LIGHT MODULATION SYSTEM USING PHASE CONTROLLED SYNOHRONOUS MOTORS Filed April 26, 1963 2 Sheets-Sheet 2 N N I\ F/yv '- INVENTOR flf/v/s Mmvxraawjwms ATTORNEYS United States Patent 3,213,377 EIGHT MQDULATION SYSTEM USING PHASE CUNTROLLED SYNCHRONOUS MOTORS Denis Manlrtelow Neale, ilford, England, assignor to Illord Limited, Ilford, England, a British company Filed Apr. 26, 1963, Ser. No. 275,958 Claims priority, application Great Britain, May 4, 1962, 17,3t)7 62 6 Claims. (Cl. 328-455) This invention relates to photographic printing and more particularly to means for controlling the mean light output of an electric discharge lamp operating on an alternating current supply and used as a light source in photographic printing.

Since in the production of prints from a large number of negatives on a commercial basis negatives of widely different density levels have to be handled, it arises that if dense negatives are to be printed rapidly, then so much light must be made available for the printing that the printing times for negatives of low density, using such printing light become inconveniently short.

In handling colour negatives there are advantages in employing a light source which is a combination of both a tungsten filament lamp and a mercury vapour lamp, and it is an object of the present invention to provide means for intensity control of such a combined light source.

According to the present invention there is provided a method of controlling the mean light output of an electric discharge lamp operating on an alternating current power supply and used as a light source in photographic printing, which method comprises energising a synchronous motor from a supply bearing a variable phase relationship to said alternating current power supply and derived therefrom by connecting across said alternating current power supply a tapped impedance, connecting also across said alternating current power supply a load impedance in series with a controlled variable reactance, said synchronous motor being connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, and providing sectors rotated by said synchronous motor intercepting light from said discharge lamp at twice the frequency of the alternating current power supply and at a phase relationship thereunto determined by the setting of the variable reactance.

The invention further includes apparatus for use in carrying out the foregoing process which comprises an electric discharge lamp and means for connecting same to an alternating current power supply, a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series cross said power supply, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, and sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said variable reactance.

According to a preferred form of the invention, a second synchronous motor is connected directly to the alternating current power supply and carries further sectors intercepting light from said discharge lamp at twice the frequency of said alternating current power supply and at a phase angle which is not influenced by the setting of the variable reactance.

According to particular forms of the invention the said controlled variable reactance may be a saturable reactor or a pair of controlled rectifiers, e .g., thyratrons, connected in inverse parallel.

The said load impedance may be a tungsten filament lamp.

The invention will be further described with reference to the accompanying drawings in which:

FIGURE 1 shows a circuit for means for intensity control of a tungsten lamp.

FIGURE 2 shows an alternative circuit for means for intensity control of a tungsten lamp.

FIGURE 3 shows a circuit illustrative of the present invention.

FEGURE 4 shows one specific embodiment of the present invention, being a circuit useful for control of intensity for a combined mercury vapour lamp and tungsten lamp light source.

FIGURE 5 shows a preferred embodunent of the present invention, being a circuit useful for obtaining wider range of mean light intensity than that obtainable employing the embodiment of FIGURE 4.

FIGURES l and 2 are included for comparison purposes.

In FIGURE 1 two tungsten filament lamps are shown connected in series and supplied from a source of alternating current through a pair of silicon controlled rectifiers connected in inverse parallel. Each silicon controlled rectifier permits of current flow in one direction only. Moreover, on each half-cycle of the alternating supply neither rectifier is conducting until a firing pulse is applied by means not here shown. Thus, by controlling the phase angle at which these firing pulses are applied it is possible to control the mean-current which flows through the tungsten lamps. It will be noted that because both rectiers are non-conductive on the first part of each halfcycle of the alternating supply, adjustment as described of the mean current flowing through the lamps necessarily produces a change in phase angle of the fundamental frequency component of the alternating current flowing through the lamps. The combination of two silicon controlled rectifiers connected in inverse parallel may therefore be regarded as a form of variable reactance.

In FIGURE 2 a single tungsten lamp is shown connected to an alternating current power supply through a saturable reactor. It will be understood that adjustment of the direct current control current through the reactor by means not here shown will vary the inductance presented in series with the tungsten lamp. As a result, such adjustment produces simultaneously a variation in magnitude and phase of current passing through the lamp. In this respect the arrangements shown in FIGURES 1 and 2 are closely similar, although the degree and nature of the distortion of current waveform passing through the lamps differ substantially in the two cases.

FIGURE 3 shows a tungsten lamp connected to an alternating current power supply through a variable reactor, indicated symbolically as X, and which may in practice be either a saturable reactor or a pair of con trolled rectifiers connected in inverse parallel. To those skilled in the art, it will be evident that a motor M connected as shown between the junction of the lamp and variable reactor and a tapping on an impedance Z connected across the supply, will receive a voltage which is not greatly dependent on the setting of the variable reactor but the phase of which changes twice as rapidly as the phase of the current through the tungsten lamp. By adjusting the variable reactor X it is thus possible to control simultaneously and in a related manner the current through the tungsten lamp and the phase of the supply to the motor Vi.

The light output from an electric discharge lamp (e.g., a mercury vapour lamp) operating on an alternating current supply is modulated at twice the supply frequency. Thus a lamp operating on a 50 c./s. mains supply will produce light modulated at 100 c./s. If this modulated light is intercepted by a sectored shutter, the sectors of which intercept the light at the same frequency as the frequency of modulation, the mean intensity of light passing the shutter will depend on the phase relation between the modulation of light by the mains supply and that by the sectored shutter. By changing the phase of energisation of the shutter motor, the invention provides a change in the aforesaid phase relation and thereby a change in mean light intensity.

In some applications it may be found that the alternating current power supply produces a modulation of light output from the discharge lamp which is insufficient to provide the range of intensity control required. This difficulty may be overcome by using a second synchronous motor operated in constant phase relation to the alternating current power supply, said second synchronous motor driving a sectored shutter to intercept the light path at twice the supply frequency. In this way the modulation of the light may be increased to an extent sufficient to allow the mean light intensity to vary over a wide range by variation of the phase of energisation of the firstdescribed synchronous motor.

FIGURE 4 represents a particular embodiment of the invention in which the variable reactor comprises a pair of controlled rectifiers R connected in inverse parallel. The variable reactor is connected in series with a load 30 across an alternating current power supply 1. Load 30 comprises a filament lamp. The tapped impedance is here represented by the tapped choke 33 and the motor is shown supplied through a step-up transformer 31. Since the controlled rectifiers R introduce severe waveform distortion an additional inductance 32 and capacitor C are used to filter out some of the harmonics in the waveform applied to the motor. In the embodiment of FIGURE 4, a single synchronous motor 4 is utilized. Energisation of the motor causes rotation of the sectored shutter 5 to move sectors 6 and 7 into position to intercept light from a discharge lamp 3. Lamp 3 is a mercury vapour lamp connected in series with a ballast inductor 2 across supply 1. The mean light intensity is determined by adjustment of the variable reactor to control the phase of energisation of motor 4.

The preferred embodiment of the invention will now be described with reference to FIG. 5, in which the controlled rectifiers are thermionic gas-filled tubes, commonly known as thyratrons. The operation of the embodiment of FIGURE 4 will become obvious when considered in light of the description of the preferred embodiment.

In FIG. 5, current from an alternating current power supply, ll, passes through a ballast inductor 2, and a mercury vapour discharge lamp, 3. The light output from discharge lamp, 3, is inherently modulated at twice the frequency of the alternating current supply. A first synchronous motor, 4, rotates a sectored shutter, 5, to intercept cyclically the path, 8, of light from discharge lamp, 3. Motor 4, is a two-pole synchronous motor and shutter, 5, has two light-intercepting sectors, 6 and 7. When motor 4 is energised by an alternating current of the same frequency as the supply 1, sectors of shutter 5 will intercept the path, 8, of light from discharge lamp 3, the frequency of interception being equal to twice the frequency of alternation of the supply ll. The mean intensity of light passing shutter 5 will therefore depend upon the phase of alternating current supplied to motor 4.

The phase of alternating current supplied to motor 4 is controlled by the thyratron valves 9, 10. Said thyratrons are connected in inverse paralled and controlled by circuitry of known type. For example, the circuit ar rangement shown, enclosed by a broken line in FIG. 5, is described by Kretzmann (Industrial Electronics, p. 130, Philips Technical Library, 1953). Rectifier tubes 11, 12, rectify the alternating current outputs of transformers 13, 14, to provide negative bias voltages across capacitors 15, 16. Potentiometers I7, 18, allow adjustable proportions of the said bias voltages to be applied to the control grids of thyratrons 9, MP, through sec ondary windings 19, 20, of peaking transformer 21, and series resistors 22, 23.

One end of the primary winding 24, of peaking transformer 21, is connected to a tapping on the primary winding 25 of transformer 13. The other end of winding 24 is connected to the phase shifting network comprising variable resistance 26 and capacitors 27, 28 and 29.

Potentiometers l7 and 18 are adjusted so that, in the absence of output from transformer 21, both thyratrons 9, 110, are just inhibited from firing. On each half-cycle of the alternating current power supply 1, transformer 21 delivers a pulse which causes one of the thyratrons 9, It), to conduct until the voltage across said thyratron falls substantially to Zero. By adjusting the value of variable resistance 26, the pulses from transformer 21 are advanced or retarded in phase relative to the phase of the alternating current power supply, I. The firing angle of the thyratrons 9, It), may in this way be varied substantially from 0 to 180 of the electrical cycle of 360.

Current passed by thyratrons 9, 1t), fiows through the filament lamp 3t). As the firing angle of thyratrons 9, 10, is advanced or retarded, therefore, the intensity of light output from filament lamp 30 increases or decreases due to the increase or decrease in current passed by the thyratrons.

The phase of the fundamental component of alternating current passed by thyratrons 9, 10, will cary from 0 to substantially as the firing angle of the thyratrons is varied from 0 to Motor 4 is energised by alternating-current from an auto-transformer 31, connected through inductor 32 to a tapping on inductor 33. As the phase of alternating current passed by the thyratrons varies by 90, therefore, the alternating current energising motor 4- will vary in phase by substantially 180 but will remain substantially of constant amplitude. Capacitor 33 combines with inductor 32 to filter from the supply to motor 4 some of the harmonics of the supply frequency resulting from the switching action of thyratrons 9, 10.

Change in phase of current energising motor 4 causes the sectors 6, 7, of shutter 5 to intercept in a different phase relation the path 8 of modulated light from discharge lamp 3. Thus variation of resistance 26 produces a change in intensity of light emitted by filament lamp 30 and also a change in mean intensity of light passing shutter 5 along path 8.

To provide maximum control of mean intensity from discharge lamp 3, a second synchronous motor 34 is disposed to rotate a shutter 35 so that sectors 36, 37 of said shutter 35 intercept the path 8 of light from discharge lamp 3. Motor 34 is so mounted that, when energised by the same alternating current supply l as discharge lamp 3, one of the sectors 36, 37, intercepts the path 8 as the intensity of light emission from discharge lamp 3 passes through a minimum. As the phase of energisation of motor 4 is varied, therefore, it is possible for shutters 5 and 35 to pass through a phase relationship in which no light is able to pass along path 8 beyond shutter 5. The mean intensity of light in path 8 can therefore be controlled from a maximum value to zero by adjustment of resistance 26.

I claim as my invention:

1. Apparatus for use in controlling the mean light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply, a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series across said power supply, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, and sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said variable reactance.

2. Apparatus for use in controlling the mean light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply, a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series across said power supply, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said con trolled variable reactance, sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said variable reactance, and a second synchronous motor connected directly to the alternating current power supply which carries further sectors intercepting light from said discharge lamp at twice the frequency of said alternating current power supply and at a phase angle which is not influenced by the setting of the variable reactance.

3. Apparatus for use in controlling the mean light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply a tapped impedance connected across said alternating current power supply, a load impedance and a saturable reactor connected in series across said power supply, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said saturable reactor, and sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said saturable reactor.

4. Apparatus for use in controlling the mean light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series across said power supply, the said controlled variable reactance being a pair of thyratrons connected in inverse parallel, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, and sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating cur- Y 6 rent power supply and at a phase relationshi thereto which is determined by the setting of said variable reactance.

5, Apparatus for use in controlling the mean light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series across said power supply, the said controlled variable reactance being a pair of thyratrons con nected in inverse parallel, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said variable reactance, and a second synchronous motor connected directly to the alternating current power supply which carries further sectors intercepting light from said discharge lamp at twice the frequency of said alternating current power supply and at a phase angle which is not influenced by the se ing of the variable reactance.

6. Apparatus for use in controlling the means light output of an electric discharge lamp operating on an alternating current power supply which comprises an electric discharge lamp and means for connecting same to an alternating current power supply a tapped impedance connected across said alternating current power supply, a load impedance and a controlled variable reactance connected in series across said power supply, said load impedance being a tungsten filament lamp and said controlled variable reactance being a pair of thyratrons connected in inverse parallel, a synchronous motor connected between a tap on said tapped impedance and the junction of said load impedance and said controlled variable reactance, and sectors adapted to be rotated by said synchronous motor to intercept light from said discharge lamp at twice the frequency of the said alternating current power supply and at a phase relationship thereto which is determined by the setting of said variable reactance.

References Cited by the Examiner UNITED STATES PATENTS 1,933,831 11/33 Tuttle et al 8824 1,971,191 8/34 Lord 3l5194 X 1,989,187 1/35 Fitzgerald 315253 X 2,010,610 8/35 Simpson 315-259 X OTHER REFERENCES Electronic Fundamentals and Applications (Ryder); published by Prentice-Hall, 1959 (pages 672673 are relied on).

J HUCKERT, Primary Examiner. 

1. APPARATUS FOR USE IN CONTROLLING THE MEAN LIGHT OUTPUT OF AN ELECTRIC DISCHARGE LAMP OPERATING ON AN ALTERNATING CURRENT POWER SUPPLY WHICH COMPRISES AN ELECTRIC DISCHARGE LAMP AND MEANS FOR CONNECTING SAME TO AN ALTERNATING CURRENT POWER SUPPLY, A TAPPED IMPEDANCE CONNECTED ACROSS SAID ALTERNATING CURRENT POWER SUPPLY, A LOAD IMPEDANCE AND A CONTROLLED VARIABLE REACTANCE CONNECTED IN SERIES ACROSS SAID POWER SUPPLY, A SYNCHRONOUS MOTOR CONNECTED BETWEEN A TAP ON SAID TAPPED IMPEDANCE AND THE JUNCTION OF SAID LOAD IMPEDANCE AND SAID CONTROLLED VARIABLE REACTANCE, AND SECTORS ADAPTED TO BE ROTATED BY SAID SYNCHRONOUS MOTOR TO INTERCEPT LIGHT FROM SAID DISCHARGE LAMP AT TWICE THE FREQUENCY OF THE SAID 